The production of fuel ethanol, or other fuels and chemicals, from lignocellulosic feedstocks provides an attractive alternative to the feedstocks predominantly used to date such as corn starch, sugar cane, and sugar beets. The production of fermentation products from these latter sources cannot increase much further as most of the farmland suitable for the production of these crops is in use. Cellulose is an abundant natural polymer, so there is an enormous untapped potential for its use as a source for fuels and chemicals. Also, lignocellulosic feedstocks to be used for fuel or chemical production are inexpensive as they have limited use.
The conversion of lignocellulosic feedstocks to a fermentation product is usually carried out with a pretreatment process prior to subsequent steps carried out to liberate glucose from the cellulose contained in the feedstock. Pretreatment makes the feedstock more amenable to subsequent conversion of the cellulose to glucose carried out with cellulase enzymes. The glucose can then be converted to a fermentation product such as ethanol by yeast or bacterium using known methods.
The fermentation product is recovered, meaning that it is concentrated and/or purified from a fermented solution. If ethanol or butanol is the fermentation product, the recovery is carried out by distillation, often with further concentration by molecular sieves or membrane extraction. A product-depleted stream resulting from the recovery of the fermentation product contains components besides the fermentation product, such as unfermented sugars, organic salts, and in some instances lignin. Such product-depleted stream may further comprise a certain level of fermentation product not removed by the recovery.
However, there are numerous challenges associated with producing a fermentation product from lignocellulosic feedstock. In order for commercialization to be more widespread, it is desirable to reduce the costs associated with energy usage, acid and/or base addition, and/or salt processing.
For example, the pretreatment is usually conducted at elevated temperature, often above 160° C. This requires a significant input of energy, which negatively impacts process economics. This energy can be supplied at least in part by the combustion of lignin, which is a byproduct of the cellulose conversion process. Although the use of lignin generated during the process is advantageous in that it can reduce the use of fossil fuels, it nonetheless presents several disadvantages. For instance, separation of lignin, which is typically conducted after hydrolysis of the cellulose with cellulase enzymes, requires an additional unit operation often employing specialized filtration equipment, which adds cost. Moreover, separation of the lignin from a hydrolyzate produced by enzymatic hydrolysis can result in sugar loss, which in turn can impact the yield of the fermentation product such as ethanol. A further drawback is that the separated lignin is usually wet, often containing 50% water, thus requiring significant energy for evaporation.
Another drawback of processes to produce products from lignocellulosic feedstocks is that the chemical used to pretreat the feedstock to prepare it for subsequent enzymatic hydrolysis with cellulase can be expensive. Further, if an acid or base is used in the pretreatment, the pH of the pretreated feedstock generally needs to be adjusted to a pH that is within a range that is compatible with cellulase enzymes and/or fermentation microorganisms. Conventionally, cellulase enzymes perform best at a pH of 4 to 6, although genetically modified enzymes are being developed that perform well over a wider range of pH values. However, the addition of an acid or base to adjust the pH of the pretreated feedstock to a suitable range increases chemical consumption, thereby adding further cost to the process.
Moreover, the addition of acid or base to the pretreated feedstock can lead to the production of organic and inorganic salts. Examples of such salts include acetate, sulfate and/or sulfite salts. Inorganic salts can be especially problematic to handle, particularly in downstream processes as described below. These inorganic salts are often carried through to the product-depleted stream remaining after the recovery of the fermentation product. One method for waste water treatment is anaerobic digestion, which produces methane by microbially breaking down organic components. However, the inorganic salts, particularly sulfur-containing salts, can reduce the efficiency of anaerobic digestion. While removal of inorganic salts prior to anaerobic digestion is possible, this also adds cost.
Although the pretreatment of lignocellulosic feedstock followed by enzymatic hydrolysis is described above, the lignocellulosic feedstock may also be treated with acid or base to hydrolyze both the xylan and cellulose component of the feedstock to produce sugar without a subsequent enzymatic hydrolysis step. Such processes may use harsher conditions than acid or alkali pretreatment processes and are often referred to as dilute or concentrated acid or alkali hydrolysis processes. Notably, many of the same problems arising from the addition of relatively dilute acid and/or base during pretreatment may arise in acid or alkali hydrolysis processes as well (e.g. that are not enzymatic). Furthermore, some processes to produce sugar involve no addition or low concentrations of acid and/or base. Salts arising from the feedstock itself, however, could present problems during such processes.